1. Field of the Invention
The present invention relates to an electric discharger of a simple circuit arrangement for causing a reactor having a dielectric body to discharge by releasing an electromagnetic energy stored in an inductor from a DC power supply unit under a low voltage.
2. Description of the Related Art
Technologies for deodorization, sterilization, and toxic gas decomposition based on a plasma developed by high-voltage pulse discharges have recently been put to use. To generate such a plasma, a high-voltage pulse generating circuit capable of supplying extremely narrow pulses of a high voltage is required.
There has heretofore been proposed a high-voltage pulse generating circuit as disclosed in Japanese Laid-Open Patent Publication No. 2004-72994, for example. As shown in FIG. 6, the proposed high-voltage pulse generating circuit, generally denoted by 200, has a simple circuit arrangement including a transformer 204, a first semiconductor switch 206, and a second semiconductor switch 208 which are connected in series across a DC power supply unit 202. The first semiconductor switch 206 has an anode connected to an end of a primary winding of the transformer 204, whose other end is connected to a cathode of a diode 210. The diode 210 has an anode connected to the gate terminal of the first semiconductor switch 206.
When the second semiconductor switch 208 is turned on, the first semiconductor switch 206 is rendered conductive, applying a voltage from the DC power supply unit 202 to the primary winding of the transformer 204, storing induced energy in the transformer 204. When the second semiconductor switch 208 is thereafter turned off, since the first semiconductor switch 206 is quickly turned off, developing a sharply rising extremely narrow high-voltage pulse Po across the secondary winding of the transformer 204. Therefore, a high voltage Vo appears between output terminals 212, 214 of the secondary winding of the transformer 204.
The high-voltage pulse generating circuit 200 is capable of supplying the high voltage Vo which has a sharp rising time and an extremely short pulse duration without the need for a plurality of semiconductor switches to which a high voltage is applied.
A reactor may be connected between the output terminals 212, 214 of the high-voltage pulse generating circuit 200 for producing an electric discharge. Silent electric discharges are advantageous in that they can develop a stable nonequilibrium plasma without causing an arc discharge, and pose little limitations on the waveform of the applied voltage. A reactor for producing silent electric discharges may have a pair of electrodes with a dielectric body and a space interposed therebetween. The dielectric body may be of alumina.
An electric discharge produced by a reactor which has a dielectric body and a space interposed between a pair of electrodes will be described below with reference to FIGS. 7 through 10D.
As shown in FIGS. 7 and 8, a reactor 300 has two upper and lower alumina plates 302, 304, which jointly serve as a dielectric body 305, support plates 308 disposed between the upper and lower alumina plates 302, 304 to keep a space 306 as a constant gap between the upper and lower alumina plates 302, 304, an upper electrode 310 disposed on an upper surface of the upper alumina plate 302, and a lower electrode 312 (see FIG. 8) disposed on a lower surface of the lower alumina plate 304.
An equivalent circuit of the reactor 300 is shown in FIG. 9. As shown in FIG. 9, the equivalent circuit has a first capacitance Cc1 of the dielectric body 305 provided by the upper and lower alumina plates 302, 304 and a second capacitance Cc2 of the space 306, the first capacitance Cc1 and the second capacitance Cc2 being connected in series with each other.
A voltage (spatial discharge voltage Vc2) applied across the space 306 is unknown. However, it can be determined from the voltage (output voltage Vo) applied to the reactor 300 in its entirety and the voltage (voltage Vc1 to charge the dielectric body 305) applied across the first capacitance Cc1 according to the following equation:Vc2=Vo−Vc1
The voltage Vc1 to charge the dielectric body 305 is expressed as follows:Vc1=Q/Cc1=(1/Cc1)×∫Ic dt, where Q represents electric charge and Ic represents an electric current flowing through the reactor 300.
As shown in FIG. 9, the reactor 300 is connected between the output terminals 212, 214 of the high-voltage pulse generating circuit 200. The high-voltage pulse generating circuit 200 is normally operated as described above. After the first semiconductor switch 206 is turned off, the voltage (output voltage Vo) applied across the reactor 300 has a waveform including a forward peak voltage Vp1 and then a reverse peak voltage Vp2, as shown in FIG. 10A. The voltage applied across the dielectric body 305, i.e., the voltage Vc1 to charge the dielectric body 305, has a waveform including a forward peak voltage Vp in synchronism with the forward peak voltage Vp1 included in the waveform of the output voltage Vo, as shown in FIG. 10C.
As shown in FIG. 10D, the discharge voltage Vc2 across the space 306 is clamped to a certain positive voltage Va1 in the period of the forward output voltage Vo and then to a certain negative voltage Va2 in the period of the reverse output voltage Vo, as can be plotted based on the above equation.
As shown in FIG. 9, the space 306 in the reactor 300 is represented by an equivalent circuit comprising a series-connected circuit 316 of two zener diodes 314a, 314b having respective anode terminals connected to each other, and the second capacitance Cc2 connected parallel to the series-connected circuit 316.
The voltage (output voltage Vo) applied across the reactor 300 etc. will be described below with reference to FIGS. 10A through 10D.
When the second semiconductor switch 208 is turned on, the first semiconductor switch 206 is rendered conductive. An electric current flows through the inductance of the transformer 204, storing induced energy in the transformer 204. When the second semiconductor switch 208 is subsequently turned off at time t10, the electric current that has flowed through the inductance of the transformer 204 flows into the reactor 300.
In this initial stage, an electric current Ic flows into the second capacitance Cc2 of the space 306 in the reactor 300, charging the second capacitance Cc2 (see the broken line P in FIG. 9). When the discharge voltage is reached, the voltage applied across the space 306 is clamped to the forward discharge voltage Va1 (see FIG. 10D), and the electric current Ic flows through the series-connected circuit 316 (see the broken line Q in FIG. 9). At this time, the dielectric body 305 quickly starts being charged, storing energy.
At time t11 when the current Ic flowing in the reactor 300 becomes zero, the dielectric body 305 stops being charged, and the energy stored in the dielectric body 305 is consumed by being discharged.
Thereafter, the electric current Ic flows backwards into the electrostatic capacitance Cc2 of the space 306, charging the electrostatic capacitance Cc2 (see the broken line R in FIG. 9). When the discharge voltage is reached, the voltage applied across the space 306 is clamped to the reverse discharge voltage Va2 (see FIG. 10D), and the electric current Ic flows through the series-connected circuit 316 (see the broken line S in FIG. 9). At this time, part of the energy remaining in the dielectric body 305 is consumed by being discharged. The energy that has not been consumed by the reactor 300 flows back to the DC power supply unit 202 (see FIG. 6).
The reactor 300, which has the upper and lower electrodes 310, 312 and the dielectric body 305, i.e., the upper and lower alumina plates 302, 304, and the space 306 interposed between the upper and lower electrodes 310, 312, has difficulty increasing pulse energy per cycle, for example. Specifically, when the first semiconductor switch 206 is turned off, the electric current flowing through the inductance of the transformer 204 flows into the reactor 300, storing a large amount of energy in the dielectric body 305. The stored energy is partly consumed by the subsequent reverse discharge, and the energy which remains unconsumed flows back to the DC power supply unit 202. As a result, though the DC power supply unit 202 undergoes a large burden, no commensurate energy can be applied to the reactor 300. Stated otherwise, though the DC power supply unit 202 undergoes a large burden, the amount of energy that is consumed by the reactor 300 is small.